Injection molding is a technology commonly used for high-volume manufacturing of parts made of meltable material, most commonly of parts made of thermoplastic polymers. The injection molding process used predominantly in the industry today is intermittent, meaning that all processes occur in a sequential fashion and thus each step must be completed before the next step can begin. A plastic resin, most often in the form of small beads or pellets, is introduced to an injection molding machine that melts the resin beads under heat, pressure, and shear. The now molten resin is forcefully injected into a mold cavity having a particular cavity shape. The injected plastic is held under pressure in the mold cavity, cooled, and then removed as a solidified part having a shape that essentially duplicates the cavity shape of the mold. The process is repeated to produce multiple parts using the same mold. The mold itself may have a single cavity or multiple cavities. If more than one molten material is injected into a mold, the injection molding process is referred to as co-injection.
U.S. patent application Ser. No. 13/774,692, incorporated herein by reference, describes co-injection processes in substantially constant pressure injection molding systems. There are numerous limitations and challenges presented by both intermittent injection molding and co-injection molding. For instance, conventional intermittent injection molding lengthens the cycle time necessary to mold a part due to the need to perform each phase or step sequentially. Also, the production of large quantities of parts using intermittent injection molding requires large equipment to hold the mold closed since the clamp tonnage must simultaneously hold multiple mold cavities closed. Conventional multi-cavity intermittent injection molding systems also occupy a large footprint to accommodate the multiple mold cavities. Conventional co-injection molding presents the manufacturing challenge of maintaining synchronized flow front velocities of the materials introduced to the mold cavity in order to maintain a consistent distribution of materials in the mold cavity. Conventional co-injection molding further requires that the thickness of parts be at least 1 mm to avoid an inner layer from bursting through an outer layer. These and other limitations and challenges, limit the circumstances in which intermittent and co-injection molding processes can be used.
One approach to address the issues with intermittent injection molding is to “compression mold” molded articles. This approach involves: 1) extruding molten polymer; 2) trimming a “plug” of extruded polymer to a predetermined length (to achieve a target volume of polymer); 3) depositing the “plug” into a bottom mold cavity; and 4) compressing an upper mold half in to a bottom mold half to form a molded part. This approach can be accomplished on a continuous rotating platform which enables each step to be accomplished simultaneously, and results in very high production rates and lower costs. However, there are numerous trade-offs. First, the polymer “plug” freezes immediately when contacting the cooled bottom mold half—this results in a noticeable matte or rough surface texture on the molded part (an undesirable quality defect). Second, the molds must be very simple in design to enable the part to be molded by the compressive forces as the upper mold half approaches the bottom mold half—this dramatically limits the part designs that are possible using this molding technique.
An alternate approach is to continually feed the polymer to a plurality of mold cavities arranged in a carousel fashion about a central polymer source. In existing continuous injection molding systems of this nature that have been proposed or put into practice, it is understood that the mold cavities are disposed about the central polymer source in a planar, hub-and-spokes fashion, with the polymer source outlet or nozzle being in the same plane as the inlet of each of the mold cavities. One drawback of this arrangement is the large footprint of manufacturing floor space required to accommodate all of the mold cavities. Another drawback is the amount of energy necessary to propagate the polymer along horizontally-extending feed channels that connect the nozzle and the mold cavities. An additional drawback is the lack of ability to make real-time adjustments to melt pressure. In at least one prior disclosure of a carousel-type continuous molding system, the system had a valve gate actuator for positioning a valve pin that controllably connected the molding cavity to a shooting pot. The valve gate was operated according to a valve gate cam profile for actuation of the valve pin. Because the valve's actuation is dependent upon a cam track, the valve position is dictated by the location of a mold position as it rotates about the carousel. As such, there is no ability to adjust the melt flow to increase or decrease pressure. The only variable determining the rate and pressure by which melt flows into a given mold cavity is the extent to which the valve is open or closed, but with no ability to make fine adjustments at the location of the valve, any pressure adjustments that may be needed would have to be accomplished by adjusting the rate of output of an extruder or other source of molten polymeric material.